Why is my angular app becoming very slow after changing the data backing a mat-tree?

Question

I am implementing the angular/material tree component and having some problems. After saving changes to the tree I am resetting the data backing the tree (which works) however the app then becomes incredibly slow, expanding nodes can take about 8 seconds.

What makes this even weirder is that the code that actually manipulates the data source is run in other places, such as adding a new child to the tree - we want the UI to update, and this does not cause a problem. It is only when saving that the app becomes slow.

save(): void {
    const trees = this.flattenedTree.filter(node => this.isRootNode(node));

    this.serviceThatHasNoSideEffects.save(trees)
        .then(result => {
            this.tree = result;
            this.flattenedTree = this.getFlattenedTree();
            this.refreshTree();
        });
}

private refreshTree() {
    const data: any[] = this.flattenedTree
        .filter(x => this.isRootNode(x))
        .filter(x => x.children.length > 0 || x.id === this.focusId);

    this.nestedDataSource.data = null;
    this.nestedDataSource.data = data;

    const focusNode = this.getNode(this.focusId);
    this.nestedTreeControl.expand(focusNode);
}

private addChild(parentNode: any, childNode: any) {
    if (!this.getNode(parentNode)) {
        this.flattenedTree.push(parentNode);
    }

    if (!this.getNode(childNode)) {
        this.flattenedTree.push(childNode);
    }

    parentNode.children.push(childNode);

    this.refreshTree();
    this.nestedTreeControl.expand(parentNode);
}

EDIT:

changing refresh tree to create a completely new data source solves the slow issue (memory leak?) but not adding a child isnt displaying in the UI. Although the child is there on the flattened tree so should be displayed.

private refreshTree() {
    const data: any[] = this.flattenedTree
        .filter(x => this.isRootNode(x))
        .filter(x => x.children.length > 0 || x.id === this.focusId);

        this.nestedDataSource = new MatTreeNestedDataSource<theTreeType>();
    this.nestedDataSource.data = data;

    const focusNode = this.getNode(this.focusId);
    this.nestedTreeControl.expand(focusNode);
}

EDIT: here is the html backing it. pretty standard.

       <mat-tree-node *matTreeNodeDef="let node" matTreeNodeToggle>
            <li class="mat-tree-node">
                <button mat-icon-button disabled></button>
                {{node.uniqueName}}
            </li>
        </mat-tree-node>

        <!--when has nested child-->
        <mat-nested-tree-node *matTreeNodeDef="let node; when: hasNestedChild">
            <li>
                <div class="mat-tree-node">
                    <button mat-icon-button matTreeNodeToggle>
                        <mat-icon>
                            {{nestedTreeControl.isExpanded(node) ? 'expand_more' : 'chevron_right'}}
                        </mat-icon>
                    </button>
                    {{node.uniqueName}}
                </div>
                <ul [class.invisible]="!nestedTreeControl.isExpanded(node)">
                    <ng-container matTreeNodeOutlet></ng-container>
                </ul>
            </li>
        </mat-nested-tree-node>
    </mat-tree>

Answer 1

Although the accepted answer seems to offer some speed improvement, the solution that finally did it for me (in Angular 8) is this:

https://stackoverflow.com/a/59655114/134120

Change the line from the official examples:

<ul [class.example-tree-invisible]="!treeControl.isExpanded(node)">

to this:

<ul *ngIf="treeControl.isExpanded(node)">

so that collapsed subtrees are not loaded in the DOM at all.

Answer 2

it took me a few hours to get it working but here is the change I made:

// cdk tree that mat tree is based on has a bug causing children to not be rendered in the UI without first setting the data to null
this.nestedDataSource.data = null;
// mat-tree has some sort of memory leak issue when not instantiating a new MatTreeNestedDataSource which causes the app to become very slow
this.nestedDataSource = new MatTreeNestedDataSource<LocationHierarchyNodeDataModel>();
this.nestedDataSource.data = data;

The displaying children issue I found here: https://github.com/angular/material2/issues/11381